Deprecate non-"matrix" names in laplacian.

Use laplacian_matrix, directed_laplacian_matrix, and normalized_laplacian_matrix only and deprecate other name aliases.
Add directed laplacian matrix to docs.
Missed some name change from laplacian->laplacian_matrix
Another missing laplacian() rename.
Add test for numpy availability.
Fix some more laplacian->laplacian_matrix name changes in docs
Doc format updates and fixes.
Merge pull request #811 from hagberg/directed-laplacian

Directed laplacian